Electrolytic capacitors manufactured in the United States have conventionally comprised a capacitor section made up of a series of layers which include a first aluminum foil coated with a layer of aluminum oxide, a paper separator, a second layer of aluminum and a further layer of paper. The layers as assembled are spirally wound to form an elongated cylinder section which is wetted by an electrolyte solution. The elongated section is contained in a housing, normally of aluminum material. A layer of pitch is placed in the housing bottom to protect the capacitor from damage when the capacitor is exposed to shock and vibration forces. The upper end of the capacitor section includes first and second tabs connected respectively to the first and second layers of foil to provide anode and cathode connections. The tabs, in turn, are connected inside the housing to terminals which extend through a header which seals off the open end of the capacitor housing. A rubber gasket is normally inserted between the circumferential edges of the header and the capacitor housing, and after introduction of the section into the housing, the top end of the open housing is then crimped or rolled over to cause the header to be urged into firm contact with a shoulder on the housing to thereby provide a hermetic seal for the housing open end.
In one type of capacitor which has been available in the field for years, the capacitor housing is made of aluminum material, the header is made of phenolic or similar thermosetting material, and the electrolyte is of a glycol type. While such capacitors have performed well over the years, the changing state of the art has resulted in the need and a demand for capacitors of reduced size which are capable of withstanding the same ripple current as capacitors of larger size which use glycol electrolyte.
It is further noted that the conventional capacitor described above is limited in its use to environments in which the temperature is less than 85.degree. C. That is, the glycol type electrolyte which is used in such capacitors requires significant amounts of water, and with the exposure of such capacitors to higher temperatures, the water tends to hydrate the foils, with consequent injury to the capacitor section.
In an attempt to provide a capacitor of smaller size with operating capabilities which are at least the equivalent of the glycol capacitor, the field has turned to the use of new types of electrolytes, one example of which are the electrolytes which basically comprise a dimethyl formamide solution. Such electrolyte, in addition to having increased stability at higher temperatures, also has the capability of imparting properties to the capacitor which enable the capacitor to accept higher AC ripple currents without experiencing a heating problem (in the order of 3:1 as compared to ripple currents which glycol electrolytes can handle). Capacitors which use such electrolyte have been found to have greater long-term stability, and will operate reliably in environments of higher temperature. In addition, since the capacitor with such type electrolyte will work at higher ripple currents for a predetermined ripple current specification, the capacitor having a dimethyl formamide electrolyte may be of a smaller size than the capacitor which uses the glycol type electrolytes.
While dimethyl formamide is known to have these inherent characteristics and advantages, it has been found that the phenolic header and the rubber seals of the conventional capacitors have a short life when a dimethyl formamide electrolyte is used. In one attempt to solve such problem, certain manufacturers have turned to the use of headers made of diallyphthalate (DAP). However, the use of such header material with the dimethyl formamide electrolyte is less than satisfactory because of the tendency of the diallyphthalate material to swell after a period of use at high temperatures and the leakage and sealing problems which result as the swelling occurs.
One successful capacitor which was manufactured by the assignee of the present application uses an aluminum header having a butyl rubber gasket located between the circumferential edges of the header and the container walls. While such header and rubber material will operate successfully in temperature environments up to 105.degree. C., and do withstand the destructive effects of the DMF electrolyte, the capacitor is relatively costly, and it has been necessary to charge a premium price for capacitors of such type. The difference between the cost of an aluminum header and a phenolic header, for example, is significant. Even the smallest saving in the manufacturing cost of a capacitor, as for example, a reduction in the cost of the capacitor header, results in a significant advantage because of the relatively high volume of capacitor production.
At least one manufacturer has provided headers for smaller size capacitors made of unfilled polypropylene material. Such headers were of relatively uniform thickness except for thin projecting circumferential rings on the upper and lower surfaces, and radial rib members which projected inwardly into the housing to assist in restraining the capacitor section against movement. A similar set of ribs was located at the bottom end of the capacitor. While headers made of polypropylene material were apparently satisfactory for smaller size capacitors, headers of relatively uniform thickness made from such material for larger size capacitors (i.e., two inch diameter and larger) did not satisfactorily withstand the pressures experienced with exposure of the capacitor to temperature conditions in excess of 85.degree. C. It is temperatures in excess of 85.degree. C., which cause the greatest difficulty.
It is an object of the present invention, therefore, to provide a novel capacitor which uses an electrolyte, such as DMF, to provide operating characteristics which are equal to or better than the conventional capacitor which uses glycol type electrolyte, which is of a lower cost than the premium type capacitor presently available on the market, and which in both smaller and larger sizes will satisfactorily withstand the pressures which occur when the capacitor is subjected to temperatures of 85.degree. C.
As noted above, the conventional electrolytic capacitors use pitch material in the bottom of the container to minimize the possible damage to the capacitor section and/or tabs when the capacitor is used in environments of severe shock and vibration. While such material has performed satisfactorily, the use of the pitch material adds a significant cost to the manufacturing operation. That is, it is necessary (a) to provide dust-free storage locations for the pitch, (or dust will settle on and stick to the pitch and appear as a contaminant in the finished product), (b) to provide machines for heating the pitch and introducing the pitch into the capacitor housing, and (c) to provide storage room for periods in the order of twenty-four hours for the capacitors as filled with pitch to cool down for room temperature testing. The use of pitch or other potting compound is a disadvantage that it occupies space that might otherwise be used to reduce the rate of pressure build-up within the capacitor during use. For example, pitch may take up to one-third or one-half of the available space.
It is a specific object of the invention to provide a capacitor in smaller and larger sizes which uses electrolytes, such as DMF, which is capable of passing shock and vibration tests established in the field without using pitch or other potting compound as a retainer mechanism, and further which may be manufactured at a significant reduction in the cost of manufacture for the previous type capacitors, and which performs satisfactorily in temperature environments in excess of 85.degree. C.